The present invention relates to cosmetic and dermatological emulsions, in particular skin care cosmetic and dermatological emulsions. In an advantageous embodiment, the present invention relates to a use which permits the stability of emulsions, in particular of O/W emulsions to be increased.
The outermost layer of the epidermis, the Stratum corneum (horny layer), is an important barrier layer and therefore of particular significance inter alia for protecting against environmental influences and drying out. As a result of contact with the environment, the horny layer is continually worn away and must therefore be continuously renewed.
A model for the skin which is widely used today in the expert field depicts the Stratum corneum as a two-component system, similar to a brick wall (bricks and mortar model). In this model, the corneocytes (horn cells) are the bricks and the complex lipid membrane in the intercellular spaces is the mortar.
Apart from its barrier effect against external chemical and physical influences, the epidermal lipids also contribute to the holding together of the horny layer and influence the skin smoothness. In contrast to sebaceous gland lipids, which do not form a continuous film on the skin, the epidermal lipids are distributed over the entire horny layer.
The extremely complex interaction of the moisture-binding substances and of the lipids in the upper layers of the skin is very important for regulation of skin moisture. Cosmetics thus usually comprise water-binding substances in addition to balanced lipid mixtures and water.
As well as the chemical composition, the physical behaviour of these substances is, however, also of importance. The development of highly biocompatible emulsifiers and surfactants having liquid-crystalline properties is thus desirable. Products formulated therewith support the liquid-crystalline organization of the intercellular lipids of the Stratum corneum and thus improve the barrier properties of the horny layer. It is particularly advantageous if the molecular constituents of such products consist of substances which are naturally occuring in the epidermis.
The main role of cosmetic skin care is taken to be the strengthening or restoration of the skin""s natural function as a barrier against environmental influences (e.g. dirt, chemicals, microorganisms) and against the loss of endogenous substances (e.g. water, natural fats, electrolytes).
If this function is disturbed, it can lead to increased resorption of toxic or allergenic substances or to attack by microorganisms and, consequently, to toxic or allergic skin reactions.
Another aim of skin care is to replenish the fats and water lost from the skin as a result of daily washing. This is particularly important when the natural regeneration ability is inadequate. In addition, skincare products should protect against environmental influences, in particular against sun and wind, and delay skin ageing.
Medical topical compositions normally comprise one or more medicaments in an effective concentration. For the sake of simplicity, reference is made to the legal provisions of the Federal Republic of Germany (e.g. Cosmetics Regulation, Foods and Drugs Act) for a clear distinction between cosmetic and medical use and corresponding products.
Customary forms in which cosmetics are applied are emulsions, i.e. metastable two- or multi-phase systems in which the individual phases are present in the liquid state. The most common emulsions are O/W and W/O emulsions. Less common application forms are multiple emulsions, i.e. those which in the droplets of the dispersed (or discontinuous) phase for their part comprise droplets of a further dispersed phase, e.g. W/O/W emulsions and O/W/O emulsions.
In order to be able to ensure the metastability of emulsions, interface-active substances, i.e. emulsifiers, are generally necessary.
The use per se of customary cosmetic emulsifiers is completely acceptable. Nevertheless, emulsifiers, as any chemical substance, may in certain cases cause allergic reactions or reactions based on oversensitivity of the user.
For example, it is known that in some people having particularly sensitive skin certain light dermatoses are triggered by certain emulsifiers and the simultaneous action of sunlight.
It is possible to prepare emulsifier-free preparations which have, for example, in an aqueous phase, dispersed oil droplets, similar to an O/W emulsion. A prerequisite for this may be that the continuous aqueous phase has a gel backbone which stabilizes the dispersed phase and other conditions besides. Such systems are sometimes called hydrodispersions or oleodispersions depending on which is the disperse phase and which is the continuous phase.
For cosmetic technology, it is how ever neither necessary no r possible to dispense with emulsifiers altogether, especially since there is a certain choice of particularly mild emulsifiers. However, the prior art lacks a satisfactorily broad range of such emulsifiers which would then also significantly broaden the application spectrum of correspondingly mild cosmetic preparations which are tolerated by the skin.
An object of the present invention was thus to provide cosmetic and dermatological p reparations having excellent skin care properties.
A disadvantage in particular of O/W emulsions is frequently their inadequate stability to relatively high electrolyte concentrations, which manifests itself in phase separation. This can occasionally lead to problems, even in the case of W/O emulsions, although it is by no means as important here as in the case of O/W systems. Whilst these can often be remedied to a certain extent through appropriate choice of the emulsifier system, other disadvantages, however, arise just as often.
On the other hand, it is often desirable to use certain electrolytes in order to be able to utilize their other physical, chemical or physiological properties.
The concentrations of all the constituents of a cosmetic or dermatological preparation are usually given in units such as % by weight, mol-% and the like. In view of their dissociation into cations and anions, which takes place to a greater or lesser extent, often in several dissociation stages, it appears more advantageous for the description of the present invention and its technical background, to start from the ionic strength of a given electrolyte in its solution. The ionic strength I of an electrolyte solution is defined as   I  =            1      2        ⁢                  ∑        i            ⁢                        c          i                ⁢                  z          i          2                    
in which ci is the concentrations of the individual types of ion (in mol/l) and zi are their charges. The physical unit of ionic strength is that of a concentration (mol/l).
For example, a 1% strength (=0.17 molar) sodium chloride solution has an ionic strength I=0.17.
Another object of the present invention was thus to discover ways of producing cosmetic or dermatological emulsions, in particular O/W emulsions, which are stable to increased electrolyte concentrationsxe2x80x94or increased ionic strength.
It was also an object of the present invention to provide preparations which significantly improve the condition of the skin, in particular skin roughness.
Surprisingly, it has been shown, and this is where the solution to these problems lies, that cosmetic and dermatological emulsions having at least one aqueous phase, comprising
(a) an effective amount of one or more interface-active substances, selected from the group consisting of alkyl glucosides, which are characterized by the structural formula 
xe2x80x83in which R is a branched or unbranched alkyl radical having from 4 to 24 carbon atoms,
(b) where in at least one of the aqueous phases one or more electrolytes is present in dissolved form, the ionic strength of the aqueous phases in which the electrolyte(s) is/are present in dissolved form being at least 0.075 mol/l,
(c) and where {overscore (DP)} is an average degree of glucosylation of up to 2,
remedy the prior art disadvantages.
It was thus not foreseeable by the person skilled in the art that the preparations according to the invention would
be more effective moisture-donating preparations,
better promote skin smoothing,
be characterized by better care action,
be better vehicles for cosmetic and medical-dermatological active substances
have higher stability to decomposition in oil and water phases and
be characterized by better biocompatibility than prior art preparations.
EP-A-0 629 396 indeed describes stable cosmetic emulsions which are characterized by a content of alkyl glucosides. According to claim 6 loc. cit., the preparations can also contain salts. The ionic strengths of the electrolytes stated therein, which are specifically disclosed in the examples and based on the weight concentrations, are 0.046 mol/l (for sodium hydroxide in Example 1), 0.023 moll (for sodium hydroxide in Example 2) and 0.03 mol/l (for Na2H2EDTA in Example 5) and thus much too low, to suggest the teaching according to the invention.
In EP-A-0 629 396, Example 6, yet another stable cosmetic emulsion containing an electrolyte having an ionic strength of 0.023 mol/l (for sodium hydroxide) is given, this Example 6 referring to preparations likewise having a content of alkyl glucosides, but according to WO 92/06778. The latter specification does not mention the use of electrolytes at all. Consequently, the prior art could not pave the way to the present invention.
The invention also relates to the use of cosmetic and dermatological emulsions, in particular O/W emulsions having at least one aqueous phase, comprising
(a) an effective amount of one or more interface-active substances, selected from the group consisting of alkyl glucosides, which are characterized by the structural formula 
xe2x80x83in which R is a branched or unbranched alkyl radical having from 4 to 24 carbon atoms,
(b) where in at least one of the aqueous phases one or more electrolytes is present in dissolved form, the ionic strength of the aqueous phases in which the electrolytes) is/are present in dissolved form being at least 0.075 mol/l,
(c) and where {overscore (DP)} is an average degree of glucosylation of up to 2,
for skin care.
The invention also further relates to the use of one or more interface-active substances, selected from the group consisting of alkyl glucosides which are characterized by the structural formula 
in which R is a branched or unbranched alkyl radical having from 4 to 24 carbon atoms, and where {overscore (DP)} is an average degree of glucosylation of up to 2, for achieving or increasing the stability of emulsions to the presence of electrolytes, in particular for achieving or increasing the stability of O/W emulsions to the presence of electrolytes.
The invention relates in particular to the use of one or more interface-active substances, selected from the group consisting of alkyl glucosides which are characterized by the structural formula 
in which R is a branched or unbranched alkyl radical having from 4 to 24 carbon atoms, and where {overscore (DP)} is an average degree of glucosylation of up to 2, for achieving or increasing the stability of emulsions to the presence of electrolytes, in particular for achieving or increasing the stability of O/W emulsions to the presence of electrolytes, in which case the emulsions, in at least one of the aqueous phases, contain one or more electrolytes in dissolved form, and the ionic strength of the aqueous phases in which the electrolyte(s) is/are present in dissolved form is at least 0.075 mol/l.
The {overscore (DP)} value represents the degree of glucosylation of the alkyl glucosides used according to the invention and is defined as       DP    _    =                                          p            1                    100                ·        1            +                                    p            2                    100                ·        2            +                                    p            3                    100                ·        3            +      ⋯        ⁢          xe2x80x83        =                  ∑        i            ⁢                                    p            i                    100                ·        i            
p1, p2, p3 . . . and pi represent the proportion of the products glucosylated once, twice, three times . . . i times in percentages by weight. According to the invention, products with degrees of glucosylation of 1-2 are advantageous, those with degrees of from 1.1 to 1.5 are particularly advantageous and those having a degree of glucosylation of about 1.3 are very particularly advantageous.
The {overscore (DP)} value takes into account the fact that alkyl glucosides are usually mixtures of mono- and oligoglucosides, depending on the preparation. According to the invention, a relatively high content of monoglucosides, typically in the order of magnitude of 40-70% by weight, is advantageous.